Silicon carbide-to-metal joint and method of making same

ABSTRACT

A method of preparing a surface of a silicon carbide article for joining by metal brazing to metal by micro-roughening that surface of the silicon carbide which is to be brazed, applying catalyst consisting essentially of aqueous PdCl 2  directly to the micro-roughened surface, in the absence of pretreatment of said surface with stannous chloride, and electrolessly depositing a thin layer of metal selected from the group consisting of Ni, Cr, Au, Ag and Cu on said catalyst-treated surface.

This is a continuation of application Ser. No. 866,815, filed July 14,1986, now abandoned, which is a division of application Ser. No.692,944, filed 01-17-85, now U.S. Pat. No. 4,610,934.

This invention relates to an easy brazable ceramic article, a method forpreparing a ceramic surface for joining by metal brazing to a metal anda brazed ceramic-to-metal joint. More particularly, this inventionrelates to silicon carbide structural ceramic articles which are easilybrazable and methods for preparing same and brazed joints between sucharticles and metals.

As used herein, the term "silicon carbide" includes any siliconcarbide-based structural ceramic material including composite materials.

Structural silicon carbide articles have recently become of highinterest as replacements for articles of metal and metallic alloys instructural applications such as engine components where a combination ofstrength, temperature resistance, oxidation resistance, hardness, wearresistance and low specific gravity are desired. However, knownstructural silicon carbide materials are not a panacea in allapplications due to high cost, difficulty of fabrication, low ruptureresistance and low mechanical impact resistance compared to metals andmetallic alloys, particularly at lower operating temperatures. Thus,pragmatically, requirements of many applications would be best filled byuse of articles having portions made of a silicon carbide and otherportions made of metal or metallic alloy. These portions could be joinedby brazing. However, structural silicon carbide and metals exhibitdissimilar physical properties which make known brazing techniques oflittle use. For example, structural sintered silicon carbide has acoefficient of thermal expansion which is about one-half that of steel,a Young's modulus about twice that of steel and a strain to failure ofless than 0.1 percent contrasted with several percent for steel.Additionally, silicon carbide is not wetted by common brazing alloys.Prior to the present invention, no technique was available by whichsilicon carbide could be joined to steel using common brazing alloys.

U.S. Pat. No. 2,871,139 to Wein discloses a silvering process fordeposition of silver upon non-metallic objects such as glass for theproduction of mirrors, in which the surface to be silvered is carefullycleaned by treatment first with a caustic alkali solution, or nitricacid solution, to remove all grease, followed by rinsing with distilledwater.

U.S. Pat. No. 3,296,012 to Stalnecker discloses an improvement in theadhesion of copper deposited by electroless or chemical deposition onceramic surfaces. To obtain sufficiently strong anchorage, instead ofbeing merely roughened or chemically etched, the surface of the ceramicmust be leached so as to provide it with a sub-microscopically poroussurface.

U.S. Pat. No. 3,690,921 to Elmore discloses a method for metal-platingalumina ceramic substrates, in which the substrate is prepared bycleaning with a hot alkaline solution, rinsing with water, immersing thesubstrate in concentrated alkali metal hydroxide solution and heating toa temperature sufficient to remove water from the solution and therebydeposit the solid alkali metal hydroxide on the surface, followed byfurther heating to a temperature above the melting point of thedeposited alkali metal hydroxide for a time sufficient to cause themolten alkali metal hydroxide to alter the surface by etching both thealumina and the binder in the substrate and thereafter cooling, rinsingand neutralizing the alkali metal hydroxide. Alternately, the clean, drysubstrate may be directly immersed in molten alkali metal hydroxide andrinsed as described before. The so treated substrate may then besubjected to electroless plating. The metal film is said to be stronglybonded to the ceramic substrate. The examples contained within thisdocument indicate the method to be applicable to alumina ceramicsubstrates as well as zircon, beryllia, steatite or silicateglass-ceramics.

U.S. Pat. No. 4,109,050 to Meehan et al discloses a surface treatmentmethod for making silicon-based ceramic composites having reducedtendency to form complex silicides when in contact with high performancemetals or metallic alloys at elevated temperatures. The method isapplicable to composites of silicon carbide-silicon. The method includesetching the surface of the silicon-based ceramic to effect removal ofsurface silicon, substantially filling the cavity resulting from suchremoval with an inorganic oxide mixture and firing the treatedsilicon-based ceramic to a temperature of up to 1250° centigrade,resulting in conversion of the inorganic oxide mixture to an adherentceramic coating. Etching is accomplished by any of various standardtechniques with a suitable etchant, e.g., mixtures of hydrofluoric andnitric acid. The etched surface is thereafter treated with an inorganicoxide blend of, for example, aluminum oxide and silicon oxide such askyanite, ball clay, kaolin, etc.

U.S. Pat. No. 4,135,012 to Yao-Sin Su discloses chemical treatment ofthe surfaces of zirconia-base ceramics to produce micropitting ormicrocratering to enable durable adhesion of noble metal coatings by amechanical keying or bonding of the coatings to the surface. The processincludes at least five steps: contacting the smooth surface of thezirconia-base ceramic with a liquid leachant selected from concentratedsulphuric acid, ammonium bisulphate, alkali metal bisulphate andmixtures thereof, at a temperature of at least about 250° centigrade,for a time sufficient to effect micro-pitting and/or micro-cratering;removing the leached surface from contact with the leachant; contactingthe leached surface with hydrochloric acid to effect removal of aresidue thereon comprising sulphate of metal elements includingzirconium in a ceramic; removing the leached surface, which is free ofsulphate residue, from contact with the hydrochloric acid; rinsing theleached surface with water to remove acid residue therefrom. The metalcoating (platinum) can be applied by any suitable technique knownheretofore.

U.S. Pat. No. 3,011,920 to Shipley, Jr. discloses a method ofelectroless metal deposition in the form of an adherent metal coating,particularly on plastic panels, for the manufacture of printed circuits.The clean substrate is catalyzed by treatment with a bath containingcolloidal particles of a catalytic metal and thereafter plating thesubstrate by treatment with a known deposition solution, e.g., a salt ofnickel, cobalt, copper, silver, gold, chromium or members of theplatinum family, and a reducing agent therefor, the catalytic metalbeing one known to catalyze the desired deposition. Superior results areclaimed from use of colloidal solutions of a desired catalytic metal,followed by introduction into the appropriate plating bath.

U.S. Pat. No. 3,057,445 to Bronnes discloses a metal-to-ceramic seal andmethod of making same. The invention particularly relates to seals whichare vacuum tight and may be employed in the construction of evacuated orgas filled envelopes for electrical devices. The structure of the sealincludes a semi-conductive oxide material bonded to a ceramic member,the semi-conductive oxide having metal particles dispersed therein, ametal coating tightly bonded to the surface of the semi-conductivelayer, which metal coating may then be secured a metal member bybrazing. The seal is obtained by applying to the surface of the ceramicmember a mixture of two oxides including an easily reduceable oxide anda difficulty reduceable oxide. Sealing is effected by first firing in anoxidizing atmosphere to cause the oxidic mixture to wet the ceramic andupon cooling effect a strong bond therewith. Thereafter, the assembly isfired a second time in a reducing atmosphere, causing partial reductionof the difficulty reduceable component of the mixturte and formation ofa semi-conducting matrix, throughout which is dispersed metal particlesof a completely reduced, easily reduceable component.

U.S. Pat. No. 3,326,719 to Beltzer et al describes a metal coatingmethod for non-conductive porous and non-porous substrates and to suchmetal coated non-conductive substrates. Steps of the process includedying the ceramic substrate with a reduced dye, or reducing the dyeafter it is applied, treating the dyed substrate with a noble metal,thereby making a thin, discontinuous metal coating on the dyed area,followed by contacting the substrate with a solution comprising a metalsalt selected from the group consisting of copper, nickel and silver,resulting in formation of a continuous metal coating. The porosity ofthe substrate after metal coating is substantially the same as beforecoating. The thickness of the coating can be controlled by the number oftimes the catalytic reduction is carried out. Once a continuous layer ofmetal is formed on the substrate, other metals can be electro-depositedusing the metal surface substrate as a cathode in conventionalelectro-deposition.

U.S. Pat. No. 3,551,997 to Etter discloses a method for electrolessplating and brazing of insulating or semi-conducting members. Typically,the ceramic member has a thin metal layer of molybdenum over which iselectrolessly plated a cobalt-copper alloy. Any substrate having anelectrically conducting surface may be used to receive the electrolesslydeposited cobalt-copper alloy. Details as to metallizing of thesubstrate are not provided.

U.S. Pat. No. 3,620,799 to Hoelscher et al describes a method formetallizing a ceramic body and to a ceramic-to-metal joint. Metallizingis accomplished by coating a surface of the ceramic body with acomposition consisting essentially of 10 to 90 weight parts uncoatedmolybdenum or tungsten metal particles, 4 to 80 weight parts molybdenumor tungsten metal particles which have an adherent coating of nickel,iron, and/or cobalt metal, and 2 to 27 weight parts ceramic sinterpowder consisting essentially of one or a combination of amagnesium-aluminum-silicate, a calcium-aluminum-silicate and amanganese-silicate. The coated body is then heated at metallizingtemperature in the range of about 1100°-1500° centigrade in reducingatmosphere to produce a sintered metal coating on the ceramic body.Thereafter, the coated ceramic body may be brazed or soldered to a metalbody or another metallized ceramic body.

U.S. Pat. No. 3,874,069 to Ingleby describes a method of bonding asilicon carbide body to a metal part by treatment of the surface of thesilicon carbide body to remove free silicon therefrom, followed byelectroplating with the metal. The electroplate is then bonded to themetal part. Free silicon may be removed by leaching the silicon carbidebody in boiling 30 percent caustic soda for several hours. A standardnickel plating solution is satisfactory; copper is an alternative. Thesilicon carbide body itself is employed as a cathode during nickelplating. Electroplating is continued until the required thickness hasbeen obtained, for example, between 0.020 and 0.050 inch. Free siliconmay also be removed by immersion of the body in a hydrofluoric/nitricacid mixture.

U.S. Pat. No. 4,199,408 to Sherman discloses a method of forming aconductive pattern on projections of a ceramic substrate. The methoddescribed may include a conventional electroless plating sequence inwhich the substrate is treated with a sensitizing solution, e.g.,aqueous SnCl₂, an activating solution, e.g., aqueous PdCl₂, followed byimmersion in an electroless metal deposition solution. This sequence isdescribed as a conventional electroless sequence, well known to thoseskilled in the electroless metal deposition art.

Kennametal Inc. Publication A80-184(10)FO, entitled "Designing WithKennametal," discloses brazed metal carbide, e.g., titanium carbide, tosteel or other base alloys including special surface treatment tofacilitate wetting by brazing alloys. The nature of this special surfacetreatment is not disclosed. Use of a sandwich braze in which a coppershim is brazed to each of the metal carbide and steel parts is disclosedfor relief of strain otherwise induced during brazing due to dissimilarthermal expansion coefficients of the parts joined by brazing.

SUMMARY OF THE INVENTION

This invention provides a method for preparing a silicon carbide surfacefor joining by metal brazing to a metal comprising micro-roughening thatsurface of the silicon carbide which is to be brazed, applying acatalyst solution to the micro-roughened surface, electrolessly applyinga thin layer of metal selected from Ni, Cr, Au, Ag and Cu, and applyinga compliant layer of metal over the thin metal layer.

The present invention also provides an easily brazable silicon carbidearticle comprising a silicon carbide substrate having a thin, adherent,electrically conductive metal layer overlying the substrate and acompliant metal layer overlying the thin metal layer.

The present invention also provides a brazed ceramic-to-metal jointcomprising a silicon carbide article, a metal article, a thin layer ofmetal adherent to the ceramic article, a compliant layer of metaloverlying and adherent to the thin layer and a layer of brazing metaloverlying the thin layer and joining the ceramic article to the metalarticle.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will become more fully apparentfrom the detailed description that follows, in conjunction with theaccompanying drawings in which:

FIG. 1 is a cross-sectional view of a brazed silicon carbideceramic-to-metal joint;

FIG. 2 is a side-elevational view, partially in section, of anembodiment of a joint between a silicon carbide shaft and a metal shaft;

FIG. 3 is an elevational view, partially in section, of a test specimenused to determine the effectiveness of brazed ceramic-to-metal joints.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, there is illustrated a brazed ceramic-to-metaljoint shown generally at number 10. Joint 10 includes a silicon carbideceramic substrate 12 having a surface 13 which has been micro-roughened.Micro-roughening should be of the order of grain size of the ceramicsubstrate or smaller; i.e., in the micrometer range. A thin metal layerthat is highly adherent to surface 13 overlies surface 13. A compliantlayer 16 of a metal overlies thin metal layer 14. Overlying compliantlayer 16 is layer 18 of a conventional brazing metal. The term "brazing"as employed herein is intended to include any metal or metal alloy whichis used to join two or more metallic components by heating to themelting point of the braze which is lower than that of the componentmaterials to be joined. In joint 10, braze 18 joins the compliant metallayer 16 with the metal substrate 20.

As depicted in FIG. 2, the brazed joint of FIG. 1 may be employed tojoin a silicon carbide shaft 24 to a metal shaft 30. The silicon carbideshaft 24 includes an end portion 26 of reduced diameter which isinserted into hollow end 32 of metal shaft 30. The end portion 26 ofshaft 24 has been treated so as to provide an etched surface comparableto surface 13 depicted in FIG. 1, to which is applied a thin adherentmetal layer 14 by electroless deposition. Over the thin adherent metallayer 14 is applied a compliant layer 16 of copper of about 0.003 inchthickness. Brazing layer 18 is applied over layer 16, preferably in theform of a thin foil which is wrapped about compliant layer 16, over theend portion 26 of ceramic shaft 24. This assembly is thereafter insertedinto the hollow end 32 of metal shaft 30. Bonding is thereafteraccomplished by heating, e.g., by torch, electrical induction heating orheating in a conventional furnace.

Preferably, the hollow end 32 of metal shaft 30 is chamfered or taperedat its end 34 to a thin edge to minimize stress in ceramic shaft 24 atthis location. Failure to provide a thinned end 34 of metal shaft 30 mayresult in premature failure of the end portion 26 of ceramic shaft 24 byfracture adjacent the end of the metal shaft 30.

Preferably, the end portion 26 is joined to ceramic shaft 24 through atapered shoulder 25 with all junctions being radiused to minimizeconcentration of stresses in the ceramic shaft 24.

The present invention also relates to a readily brazable ceramic article15 which includes, as shown in FIG. 1, a silicon carbide ceramicsubstrate 12 having a surface 13 thereof micro-roughened by etching.Adhered to surface 13 is a thin metal layer 14, e.g., of nickel.Overlying the thin metal layer 14 is a compliant layer 16 of metal,e.g., copper. Overlying the compliant layer 16 is preferably anotherthin layer 19 of a metal such as nickel to prevent oxidation of thecompliant layer 16 prior to brazing. Article 15, which includes anoutermost optional anti-oxidation layer 19, may be stored in openatmosphere for an indefinite length of time without loss of itscapability to be readily brazed to a metal substrate.

The manner in which a silicon carbide ceramic substrate is prepared forjoining to a steel substrate, and thereafter joined to such steelsubstrate, will now be described in detail by way of an example and withreference to FIGS. 2 and 1.

A sintered silicon carbide shaft 24 is to be joined to a steel shaft 30.The procedure comprises six major steps:

1. proper sizing of the attachment area, i.e., end 26, of the ceramicshaft 24 relative to the metal shaft 30;

2. preparation of the surface of the ceramic shaft 24 preparatory toapplication of a thin adherent metallic coating 14;

3. application of a compliant layer 16 of metal over the adherent thinmetallic coating 14;

4. final machining, if needed, of the outermost metal layer 16 to therequired final dimension;

5. application of a brazing metal or alloy layer 18;

6. actual brazing by application of heat.

Step 1--Sizing of the attachment area.

The end portion 26 of silicon carbide shaft 24 should be about 0.006inch less outside diameter than the inside diameter of the hollow end 32of steel shaft 30. This may be accomplished by diamond grinding of theend portion 26 of the ceramic shaft 24 in a lathe.

Step 2--Cleaning and micro-roughening the ceramic attachment area.

Micro-roughening of the attachment area is needed to obtain a surfacehaving a topography capable of receiving in interlocking manner the thinmetal layer. Micro-roughening is best accomplished by chemical etchingalthough mechanical means may be employed. After proper dimensioning,the end portion 26 of the silicon carbide shaft 24 must be cleaned toremove any contaminants such as oil or grease which may inhibit etching.Any suitable degreasing solvent may be employed. GENESOLV DMC™, a methylchloride-based solvent available from Allied Chemical, has been found tobe effective. If any length of time has expired subsequent to thedegreasing step, a final wash with acetone is performed immediatelyprior to etching. To avoid thermal shock, the ceramic shaft is preheatedto the approximate temperature of the etchant bath. This additionallyserves to prevent cooling of the etchant bath below its activetemperature. A preferred etchant bath consists of one part of potassiumhydroxide and one part of potassium nitrate heated to an activetemperature which is approximately 500°-600° C. This molten-salt bath iscontained in a nickel crucible which may be heated with a bunsen burner.That the bath is sufficiently heated to be in an active state isindicated by the presence of an amber color of the etchant solution whenat the appropriate temperature.

The end portion 26 of the silicon carbide shaft is immersed in themolten salt mixture for a time sufficient to cause micro-roughening ofthe surface as depicted at numeral 13 in FIG. 1. This is normallyaccomplished upon six to eight minutes immersion in the molten saltbath. That sufficient etching has occurred may be virtually confirmedupon removal of the part from the molten salt bath and removal of theetchant from the surface of the part. Normally, as manufactured,sintered silicon carbide parts, even after diamond grinding, have arather shiny surface. This surface, upon sufficient etching, is changedto a matt finish.

While the nature of the etching mechanism is not completely understood,it is believed that a chemical reaction occurs with the silicon carbide.For this reason, extreme caution is necessary in utilizing the moltenbath. If the molten salt bath is overheated, it may become extremelyreactive causing it to be violently thrown out of the crucible. Afterthe silicon carbide part has been immersed in the molten salt bath for asufficient time, it is removed and allowed to slowly cool in air.

An alternative etching procedure includes placing the silicon carbidepart in a crucible at room temperature, adding crystalline roomtemperature etchant, heating the crucible and its contents in a closedfurnace to melt the etchant and effect etching of the ceramic part,shutting off the furnace and letting the crucible and its contents coolin the closed furnace until below the boiling point of water.

Another etching procedure includes painting a saturated aqueous solutionof etchant onto the silicon carbide part, heating the painted part todrive off the water, further heating the dried painted part to about600° C., observing the etchant coating fuse and thereafter return to asolid state thereby indicating the end of reaction with the siliconcarbide part, and cooling the part to below the boiling point of water.This procedure may have to be repeated to cause sufficient etching.

To avoid thermal shock, after cooling to below the boiling point ofwater, and preferably to well below the boiling point of water (forexample, room temperature), the etchant, which has adhered to the endportion 26 of the silicon carbide shaft 24 is washed with tepid waterand acetone and dried in air. The etched surface 13 should not betouched by human hands prior to metallizing.

Step 3--Metallizing the attachment area.

It is necessary to establish an adherent metallic layer. Because manysilicon carbide materials are poor electrical conductors and thus cannotbe directly and uniformly electroplated, the application of a thinadherent metallic layer 14 is accomplished by the catalyzed electrolessdeposition of a metal. The end portion 26 of silicon carbide shaft 24 tobe catalyzed is preheated to about 80°-90° C. Thereupon, the catalyst isevenly applied. The catalyst may be applied by painting, spraying orimmersion. To date, best results have been obtained with painting. Thecatalyst is a saturated solution/dispersion of PdCl₂ devoid of stannouschloride in distilled or deionized water. Less than 0.0001 gram of thiscatalyst per square inch is necessary to catalyze the surface of thesilicon carbide part. The catalyst solution may be prepared by addingparticulate palladous chloride to distilled or deionized water. Thecatalyst solution is maintained at or above room temperature. Asufficient amount of palladous chloride particulate is added so that thesolution is saturated or super-saturated at room temperature. A suitablePdCl₂ powder is available from Alfa Products, Thiokol Neutron Division,Danvers, Mass.

After application of the catalyst, the treated silicon carbide part isallowed to dry. This may be accelerated by placing the catalyzed part inan oven at 80°-90° C.

The adherent thin metal layer 14 may be any that is easily electrolesslyplated. Suitable metals include nickel, gold, silver and copper;preferred among these is nickel.

An eminently suitable nickel layer may be had by using an electrolesssolution known as PM-980, available from the Shipley Company, Newton,Mass. PM-980 is manufactured according to U.S. Pat. No. 3,001,920. Theelectroless bath is prepared as follows: 1 part of PM-980 is added to 6parts distilled water; thereafter, the pH value is adjusted to between8.5 and 9.5 by addition of ammonium hydroxide (29 weight percent NH₃).Approximately 1/5 part is required to bring the pH into the desiredrange. The bath is heated to about 70° C. and the warmedcatalyst-treated silicon carbide shaft 24 transferred from an ovendirectly into the bath. Occurrence of electroless deposition isindicated by bubbles arising from the treated surfaces of the endportion 26 of shaft 24.

Electroless deposition is continued until a thin, highly adherent layerof metal has been deposited over the entire attachment area. When nickelis employed, best results are obtained when a thickness of less thanabout 0.0005 inch has been deposited. Excessive thickness reducesstrength of the final brazed joint. For the conditions just stated, thisoccurs in approximately ten minutes. The cylindrical surface of endportion 26 must be completely coated with the thin metal layer; this canbe visually confirmed with the naked eye. It is recommended thatconfirmation be made after about five minutes immersion in theelectroless deposition bath. Areas which are not active metal depositionsites should be retreated with catalyst prior to continuance ofelectro-deposition.

Upon completion of electroless deposition, the ceramic part should beimmediately washed with water and acetone to inhibit oxidation of thenewly plated surface.

If the compliant layer 16 of metal is to be copper electroplated in acyanide bath, the ceramic part is ready to be taken to such step.

If the compliant layer 16 is to be copper applied from an acid bath,further preparation of the end portion 26 is performed to provide a thinelectrolessly deposited layer of copper over the nickel layer. In thisinstance, the end portion 26 is first catalyzed, for example, usingShipley Company catalyst 9-F. This bath may be prepared by adding twoparts hydrochloric acid to two parts of deionized water, stirring andthereafter adding one part of Shipley catalyst 95. This bath should beused at a temperature of between 65°-100° F. The end 26 of the ceramicshaft 24 is immersed in this bath for about five minutes and thereafterremoved and rinsed in clean running water. Following this, the shaft 24is placed in an accelerator bath which may be prepared by adding onepart of accelerator 9-F to five parts dionized water. The bath should beoperated at between 70°-80° F. The catalyzed end portion 26 is soakedfor about 10 minutes; thereafter, the end portion 26 is rinsed with coldrunning water. End portion 26 is then immersed in electroless-coppersolution. A suitable electroless-copper solution is CUPOSIT Mix 328 fromShipley Company. This solution is employed in a bath of the followingcomposition: to 1 part of CUPOSIT 328-A is added one part of CUPOSIT328-B, followed by addition of 0.158 parts of CUPOSIT mix 328-C. Theoperating temperature of the bath should be about 70° F. The end portion26 of the ceramic part should be immersed in this bath for about threehours resulting in deposition of approximately 0.00009 inch thicknesscopper on the end portion 26. The plated part is thereafter washed incold running water and immediately washed in acetone to inhibitoxidation.

Step 4--Application of a compliant layer of metal.

The shaft 24 having a thin metallic layer 14, which is highly adherentto end portion 26, is wired with a copper wire adjacent tapered shoulder25. The opposite end of this copper wire is connected to the negative(cathode side) of a direct current power supply. Standard commerciallyavailable copper-phosphorus anodes are immersed in the bath whichcontains a standard copper electroplating solution. A suitable acidplating acid solution is UDYLITE™ Bright Acid Copper Plating ProcessUBAC-1, available from Udylite Corporation, Detroit, Mich.

Plating time is of course dependent upon the current density and amperesper square foot. Utilizing the solution and arrangement just described,about 40 minutes are required to plate one thousandth of an inch persquare foot at 25 amperes. To plate the end portion 26 of a shaft 24whose end portion 26 has about 3 square inches of attachment surfacearea to be plated, a current of about 300 milliamps is applied for about4.5 hours. After plating to the required thickness, the part is washedin running water, and immediately in acetone, to prevent oxidation.Preferably, the thickness of the compliant layer is from about 0.002 to0.004 inch; more desirably, the thickness of the compliant layer is0.003 inch. Suitable metals for the compliant layer include Cu, Ag, Au,Pt and brass, with Cu being preferred.

Step 5--Machining of the compliant metal layer to proper thickness.

In some instances, it may be necessary to finish machine the compliantmetal layer 16. This will most commonly occur when the end portion 26 ofthe silicon carbide shaft has the configuration of a right cylinder. Asis well known in the electroplating art, the amount of metal depositedis directly proportional to the current density, which is greatest atsharp corners such as at the end of the end portion 26. For this reason,the shaft end portion 26 is preferably formed so as to contain a taperedconical end (not illustrated) which is cut off subsequent to theelectroplating of the compliant layer 16 of metal.

After application of the requisite thickness of compliant layer 16,there is optionally and preferably applied a thin electroless nickelcoating. This thin protective metal layer 19 prevents oxidation of thecompliant copper layer 16 and provides an easily wettable surface forthe brazing metal. This optional nickel layer may be applied by paintingthe part with palladous chloride catalyst, immersing the catalyzed partin an electroless nickel bath and removing the part and rinsing it withwater and acetone and drying the rinsed part. This step is needed forbrazing a copper compliant layer 16 in air; it is not needed if inertgas or vacuum brazing are to be employed.

Step 6--Brazing.

Unless otherwise described, conventional brazing technology includingfluxes is employed where applicable. Suitable brazing alloys and fluxesare available from Lucas-Milhaupt, Cudahy, Wis. These materials have thefollowing general characteristics:

                  TABLE 1                                                         ______________________________________                                                          Melting  Flow  Specific                                     Alloy Composition Point    Point Gravity                                                                              Flux                                  ______________________________________                                        (TEC) 5% Ag-95% Cd                                                                              340° C.                                                                         395° C.                                                                      8.82   TEC                                                                    gr/cc                                        Easy- 50% Ag      625° C.                                                                         635° C.                                                                      9.45   LT Flux                               flotm 15.5% Cu             gr/cc                                                    16.5% Zn                                                                      18% Cd                                                                  ______________________________________                                    

Preferably, the brazing alloy is in the form of a thin foil which iswrapped around the end portion 26 of the silicon carbide shaft 24. Theceramic shaft 24 and the metallic shaft 30 are held in fixtures (notillustrated) with the ceramic shaft end portion 26 and brazing alloylayer 18 inserted into the hollow end 32 of the metal shaft 30.Alternatively, the brazing metal may be allowed to be drawn by capillaryaction into the remaining space between end portion 26 and hollow end32. To prevent excessive heat transfer from the silicon carbide shaftand metal shaft to be joined, the fixtures may be formed of graphite.

When a protective atmosphere, e.g., inert gas or vacuum or suitable fluxis employed, the brazing layer may also be the compliant layer. Softsolders, silver solders and copper are preferred for this purpose. As aspecific example, brazing of an electrolessly nickel plated siliconcarbide part to steel may be accomplished using copper metal in a vacuumfurnace. In this instance, electroplating of a compliant layer of copperand application of a separate brazing alloy layer may be omitted. Asanother specific example, a ceramic part including an electrolesslyplated nickel layer and an electroplated copper layer may be directlybrazed to steel by heating in a vacuum furnace.

The method just described in detail is particularly suitable for usewith ceramic substrates of sintered silicon carbide. Nickel has beenfound to be eminently suitable as an adherent layer 14 on the cleanetched surface of a sintered silicon carbide substrate 12. Because thecoefficient of expansion of nickel is much greater than that of siliconcarbide, the thickness of the electrolessly deposited nickel layershould be kept small to minimize stress. This is true respecting anymetal because of the great difference in coefficient of thermalexpansion between metals and known silicon carbide materials.

The catalyst solution for the electroless deposition of the adherentthin metal coating of nickel should be a saturated solution of palladouschloride, PdCl₂. This catalyst solution should not contain stannouschloride. Silicon carbide parts should not be treated with SnCl₂ beforetreatment with PdCl₂.

The term "compliant metal layer" refers to one which is capable ofyielding without rupture to the forces arising between the metal andceramic during the heating and cooling cycle of brazing. The compliantlayer 16 has a Young's modulus which is lower than that of steel andvery much lower than that of silicon carbide. The compliant layer 16,when of copper as aforedescribed, is about 80 percent dense. It ispossible to have a compliant layer 16 of a metal whose inherentproperties include a hardness and a Young's modulus much greater thanthose of copper. It is the characteristics of the layer 16, rather thanthose of the metal of which the layer is formed, which are critical.Suitable metals for formation of the compliant metal layer includecopper, silver, gold, platinum and brass. If lower strength andtemperature resistance is acceptable for the intended application, lowand medium temperature solders, e.g., lead/tin and lead/tin/silver/goldand the like, may be used. If the compliant metal layer is too thin in abrazed joint between a silicon carbide shaft and a steel shaft, i.e.,less than about 0.001 inch in thickness, premature failure of suchbrazed joint is likely. If the compliant metal layer is of excessivethickness, e.g., greater than about 0.004 inch in a brazed joint betweena silicon carbide shaft and a steel shaft, failure of such brazed jointis likely to occur at or near to the inherent shear strength limit ofthe metal of which the compliant layer is formed.

FIG. 3 illustrates a test specimen 40 which may be easily constructedand thereafter loaded as shown to determine the effectiveness of abrazed joint between ceramic substrate 41 and metal tube 44. Typicaldimensions may be about 3/8 inch O.D. for the ceramic cylinder 41 withthe I.D. of metal tube 44 being approximately 0.006 inch greater.

The following Table 2 demonstrates the resistance to shear stress ofvarious brazed joints utilizing a pressureless sintered silicon carbidecylinder within a steel cylinder. In each example, the silicon carbidecylinder has an outside diameter of 0.365 inch. Unless otherwisespecified, the silicon carbide cylinder was inserted into the hollowsteel cylinder to a depth of 1/2 inch and held at this depth duringbrazing. In each of examples 1-4, failure occurred due to shearingwithin the brazing material. In example 5, no failure occurred uponreaching the 10,000 pound limit of the load cell. Examples 6 and 7 wereprepared as example 5 except that the silicon carbide shaft was inserted1/4 inch into the steel cylinder and held in this position duringbrazing. In examples 6 and 7 failure occurred at more than 24,000 psi,which is above typical tensional stress or torsional stress failurevalues observed for pressureless sintered silicon carbide shafts ofsimilar diameter.

                  TABLE 2                                                         ______________________________________                                             Copper    Steel                                                               Compliant Cylinder                                                       Sam- Layer     Inside           Shear                                         ple  Thickness Diameter Brazing Stress                                        No.  (inches)  (inches) Material                                                                              (PSI) Remarks                                 ______________________________________                                        1    .001"     .373"    Kester  1744. Did not fail                                                    2.5 Ag--Pb    limit of load                                                   solder.       cell.                                   2    .001"     .373"    Kester  1744. Did not fail                                                    2.5 Ag--Pb    limit of load                                                   solder.       cell.                                   3    .003"     .375"    Kester  3230. 0.0005"                                                         5.5 Ag--Pb    electroless                                                     solder.       Ni                                                                            antioxidant                                                                   coating.                                4    .002"     .373"    Kester  3660. 0.0005"                                                         5.5 Ag--Pb    electroless                                                     solder.       Ni                                                                            antioxidant                                                                   coating.                                5    .004"     .377"    Handy & 17,440                                                                              0.0005"                                                         Harman  (Did  electroless                                                     Easyflo not   Ni                                                              silver  fail) antioxidant                                                     braze.        coating.                                6.   .003"     .375     H & H   24,700                                                                              SiC shaft                                                       Easyflo       inserted                                                                      1/4" depth                              7.   .004"     .377     H & H   25,430                                                                              SiC shaft                                                       Easyflo       inserted                                                                      1/4" same                                                                     depth.                                  ______________________________________                                    

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit or scope of the invention.

We claim:
 1. A method of preparing a surface of a silicon carbidearticle for joining by metal brazing to metal comprising:(a)micro-roughening that surface of the silicon carbide article which is tobe brazed; (b) applying catalyst consisting essentially of aqueous PdCl₂directly to the micro-roughened surface in the absence of pretreatmentof said surface with stannous chloride; and (c) electrolessly depositinga thin layer of metal selected from the group consisting of Ni, Cr, Au,Ag and Cu on said catalyst-treated surface.
 2. The method of claim 1further comprising applying a compliant layer of metal over the thinmetal layer.
 3. The method of claim 1 wherein the micro-rougheningfurther comprises exposing that surface of the silicon carbide which isto be brazed to a chemical etchant which includes molten alkali metalhydroxide; and wherein the electrolessly depositing of a thin layercomprises depositing a thin layer of Ni to less than about 0.0005 inchthickness.
 4. The method of claim 2 wherein the compliant metal layerselected from the group consisting of Cu, Ag, Au, Pt and brass iselectroplated to a thickness of about 0.002 to about 0.004 inch over apreviously deposited thin layer of Ni not exceeding about 0.0005 inchthickness.
 5. The method of claim 4 wherein the compliant metal layercomprises a layer of Cu electroplated to a thickness of about 0.003inch.
 6. The method of claim 2 further comprising applying a layer of Niover the compliant layer.
 7. The method of claim 1 wherein the articleis preheated prior to micro-roughening, said micro-roughening comprisingchemically etching in a molten bath consisting essentially of aboutequal parts of KOH and KNO₃.
 8. The method of claim 7 wherein the etchedarticle is cooled to about room temperature prior to application ofcatalyst.
 9. The method of claim 7 wherein the etched article is cooledto a temperature sufficient to avoid thermal shock upon washing withwater and is thereafter washed with water to remove any etchant adheringthereto and thereafter dried in air.
 10. The method of claim 1 whereinthe catalyst consists essentially of a saturated solution of PdCl₂,which solution is at or above room temperature during application. 11.The method of claim 9 wherein a catalyst consisting essentially of asaturated solution of PdCl₂ is applied to the previously etched anddried surface of the article.
 12. The method of claim 9 wherein thecatalyst solution is painted onto the previously dried surface of thearticle and allowed to dry before beginning electroless deposition. 13.The method of claim 1 wherein the thin layer comprises a layer notexceeding about 0.0005 inch thickness of Ni is electrolessly depositedover the catalyzed surface.
 14. The method of 3 wherein the article iswashed and dried after application of the Ni layer and thereafter iselectroplated to a thickness of about 0.003 inch with a metal selectedfrom the group consisting of Cu, Ag, Au, Pt and brass.
 15. The method of3 wherein a thin layer of Cu is electrolessly deposited over the Ni andthereafter Cu is electroplated from an acid bath to a thickness ofbetween about 0.002 and 0.004 inch.
 16. The method of claim 3 wherein Cuis electroplated from a cyanide bath directly over the Ni layer to athickness of about 0.002 and 0.004 inch.
 17. A method joining a siliconcarbide article to a metal article including the steps of:(a) chemicallyetching a surface of said silicon carbide article to micro-roughen saidsurface; (b) applying catalyst consisting essentially of aqueous PdCl₂directly to said etched surface in the absence of any pretreatment ofsaid surface with stannous chloride; (c) electrolessly applying a thinlayer of metal selected from the group consisting of Ni, Cr, Au, Ag, andCu to said catalyst-treated surface; (d) joining said electrolesslycoated surface to said metal article by positioning said electrolesslycoated surface proximate said metal article and fusing a brazing metalin the space between the coated surface and the metal article.
 18. Themethod of claim 17 wherein said brazing metal is selected from the groupconsisting of copper, alloys of lead and tin or alloys containing Ag orAu; the brazing metal while in a solid state is positioned between saidelectrolessly coated surface and said metal article; and joining iseffected by heating of said silicon carbide article, said brazing metaland said metal article together in a furnace.
 19. The method of claim 18wherein joining is effected in the presence of vacuum or inert gas. 20.The method of claim 17 wherein a compliant layer of Cu is deposited byelectroplating over said thin layer of metal and joining is effected byfusing said compliant layer.
 21. The method of preparing a surface of asilicon carbide article for joining by metal brazing comprising thefollowing sequence of steps:(a) preheating said article; (b) immersingsaid surface which is to be brazed in a molten etchant bath consistingessentially of about equal parts of KOH and KNO₃ for a time sufficientto cause micro-roughening of said surface; (c) removing said articlefrom said molten bath; (d) allowing said article to cool to temperaturebelow the boiling point of water; (e) washing said surface to remove anyremaining etchant; (f) drying said surface and without further chemicalpretreatment; (g) coating said dried surface with a saturated solutionconsisting essentially of palladous chloride; (h) allowing said surfaceto dry; (i) electrolessly depositing a layer of Ni to a thickness ofabout 0.0005 inch; and (j) washing the electrolessly deposited Ni layerand allowing it to dry.